Surface acoustic wave filter device with balanced and unbalanced terminals

ABSTRACT

A surface acoustic wave filter device includes a piezoelectric substrate, and first and second surface acoustic wave filter elements disposed on the piezoelectric substrate. Each of the surface acoustic wave filters has a plurality of IDTs disposed along the surface acoustic wave propagation direction. The first and second surface acoustic wave filter elements are arranged such that they are substantially equal in transmission amplitude characteristic within a band but different in transmission phase characteristic by about 180°. One end of each of the first and second surface acoustic wave filter elements define an unbalanced terminal, and the other end of each of the first and second surface acoustic wave filter elements define a balanced terminal.

This application is a Divisional of U.S. patent application Ser. No.09/781,126 filed Feb. 9, 2001, currently pending.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a surface acoustic wave filter, andmore particularly, to a surface acoustic wave filter having an impedancethat is different between the input and output sides and which has anunbalance-to-balance conversion capability.

2. Description of the Related Art

In recent years, the size and the weight of portable telephones arebecoming increasingly smaller. To this end, in addition to the reductionin the number of component parts and the reduction in the size thereof,development of components having a plurality of functions is beingpursued.

In view of the above situation, research is being pursued to produce asurface acoustic wave filter having a balance-unbalance conversioncapability called balun capability for use in the RF stage of portabletelephones, and some surface acoustic wave filters are used mainly inGSM.

In portable telephones, a portion extending from an antenna to abandpass filter is generally arranged in an unbalanced fashion and has acharacteristic impedance of 50 Ω. On the other hand, amplifiers or othercomponents following the filter are generally arranged in a balancedmanner and have an impedance of 150 Ω to 200 Ω. In view of the above, ithas been proposed to form a bandpass filter using a surface acousticwave filter having a capability of converting 50 Ω unbalanced impedanceto 150-200 Ω balanced impedance.

In a technique disclosed in Japanese Unexamined Patent ApplicationPublication No. 10-117123, four surface acoustic wave filter elementsare used to realize conversion from an unbalanced input to a balancedoutput. FIG. 28 shows the structure of the surface acoustic wave filterdevice disclosed in Japanese Unexamined Patent Application PublicationNo. 10-117123 cited above. In this surface acoustic wave filter device,a first surface acoustic wave filter unit 203 is constructed bycascading two surface acoustic wave filter elements 201 and 202, and asecond surface acoustic wave filter unit 206 is defined by cascading asurface acoustic wave filter element 204 and a surface acoustic wavefilter element 205 having a transmission phase characteristic that isdifferent by about 180° from that of the surface acoustic wave filterelement 204. The input/output terminals of the respective surfaceacoustic wave filter units 203 and 206 are connected in parallel or inseries so that the parallel-connected terminals define unbalancedterminals and the series-connected terminals define balanced terminals.

FIG. 29 illustrates a surface acoustic wave filter device 211 havingthree IDTs, disclosed in Japanese Unexamined Patent ApplicationPublication No. 6-204781. In this surface acoustic wave filter device211, two output-side IDTs 212 and 213 are disposed on respectiveopposite sides such that phases become opposite to each other, and theoutput terminals of the respective IDTs 212 and 213 define balancedterminals. One end of an input-side IDT 214 disposed at a centrallocation defines an unbalanced terminal. In this structure, the inputimpedance may be set to 50 Ω, and the output impedance may be set to150-200 Ω.

In the surface acoustic wave filters having the balance-unbalanceconversion capability, expansion of the passband is desired to meet therequirement of expansion of passbands in portable telephone systems. Insurface acoustic wave filters having the balance-unbalance conversioncapability, it is required that the transmission characteristics fromthe unbalanced terminal to the balanced terminals should be equal inamplitude but different by 180° in phase, over the passband. That is, animprovement in the degree of balance is desired.

However, in the surface acoustic wave filter device disclosed inJapanese Unexamined Patent Application Publication No. 10-117123, theexpansion of the passband causes the impedance of the surface acousticwave filter elements to become capacitive. This resultant capacitanceand the parasitic capacitance present between the two cascaded stagescause an impedance mismatch between surface acoustic wave filter units.This makes it difficult to achieve expansion of the passband.

Furthermore, because as many as four surface acoustic wave filterelements are used, complicated interconnections are required, and thecomplicated interconnections result in an increase in parasiticcapacitance which in turn results in degradation in the degree ofbalance. Furthermore, the use of the large number of surface acousticwave filter elements results in an increase in size which makes itdifficult to obtain a small device size. Furthermore, the use of thelarge number of surface acoustic wave filter elements results in areduction in the number of surface acoustic wave filter devices obtainedfrom each wafer, and thus results in an increase in cost.

On the other hand, in the surface acoustic wave filter device disclosedJapanese Unexamined Patent Application Publication No. 6-204781, the twoIDTs 212 and 213 have different structures so as to achieve the balancedterminals, and the locations of the IDTs 116 and 117 with respect to thelocation of the central IDT 214 are different from each other. Suchdifferences in structure or location often cause degradation in thedegree of balance. Furthermore, the series connection of the IDTs 212and 213 on the balanced terminal side results in an increase in loss dueto the resistance of the electrode fingers, which results in an increasein insertion loss in the passband.

SUMMARY OF THE INVENTION

In order to overcome the problems described above, preferred embodimentsof the present invention provide a surface acoustic wave filter devicewhich has unbalanced/balanced input/output terminals and which has awide passband and a high degree of balance.

In the surface acoustic wave filter device according to a firstpreferred embodiment, the second and third surface acoustic wave filterelements are arranged such that they are substantially equal intransmission amplitude characteristic within the band but different intransmission phase characteristic by about 180°, and at least one IDT ofthe second surface acoustic wave filter element and at least one IDT ofthe third surface acoustic wave filter element are connected to at leastone IDT of the first surface acoustic wave filter element. Thus, thesurface acoustic wave filter device can achieve the balance-unbalanceconversion capability by using the electrode connected to the firstsurface acoustic wave filter element as an unbalanced terminal and usingthe terminals connected to respective second and third surface acousticwave filter elements as balanced terminals. In this surface acousticwave filter device according to the first preferred embodiment of thepresent invention, unlike the conventional surface acoustic wave filterdevice in which four surface acoustic wave filter elements are used, thebalance-unbalance conversion capability is achieved using only threesurface acoustic wave filter elements. As a result, reductions in thesize and the cost of the surface acoustic wave filter device having thebalance-unbalance conversion capability are achieved.

Furthermore, the reduction in the number of surface acoustic wave filterelements results in a reduction in parasitic capacitance, whichsuppresses degradation in the degree of balance and which makes is easyto expand the passband.

In the surface acoustic wave filter device according to a secondpreferred embodiment of the present invention, the second and thirdsurface acoustic wave filter elements are arranged such that they aresubstantially equal in transmission amplitude characteristic within aband but different in transmission phase characteristic by about 180°, asecond IDT of the first surface acoustic wave filter element isconnected to the second surface acoustic wave filter element, and athird IDT of the first surface acoustic wave filter element is connectedto an IDT of the third surface acoustic wave filter element. Thus, thesurface acoustic wave filter device can achieve the balance-unbalanceconversion capability by using the electrode connected to the firstsurface acoustic wave filter element as an unbalanced terminal and usingthe terminals connected to respective second and third surface acousticwave filter elements as balanced terminals. In this surface acousticwave filter device according to the second preferred embodiment of thepresent invention, unlike the conventional surface acoustic wave filterdevice in which four surface acoustic wave filter elements are used, thebalance-unbalance conversion capability is achieved using only threesurface acoustic wave filter elements. As a result, reductions in thesize and the cost of the surface acoustic wave filter device having thebalance-unbalance conversion capability are achieved.

Furthermore, the reduction in the number of surface acoustic wave filterelements results in a reduction in parasitic capacitance, whichsuppresses degradation in the degree of balance and which makes is easyto expand the passband.

In the surface acoustic wave filter device according to the secondpreferred embodiment of the present invention, if the first space andthe second space are arranged such that they are different by an amountwithin the range from about 0.48λ to about 0.525λ, the amplitude balancebecomes equal to or lower than about 1.5 dB and the phase balancebecomes equal to or smaller than about 20°. As a result, it is ensuredthat degradation in the degree of balance is prevented.

If the first space and the second space are arranged so as to satisfyEquations 1 and 2, respectively, a sufficiently large bandwidth can beobtained, and degradation in the degree of balance is minimized.

Furthermore, in the surface acoustic wave filter device according to thesecond preferred embodiment of the present invention, if the first spaceand the second space are arranged so as to satisfy Equations 3 and 4,respectively, a sufficiently large bandwidth can be obtained anddegradation in the degree of balance is minimized, even when a variationin frequency due to a temperature variation is taken into account.

If the first space is within the range from about 1.72λ to about 1.83λand the second space is within the range from about 2.22λ to about2.33λ, the degradation in the degree of balance can be furthersuppressed and a sufficiently wide bandwidth can be achieved.

In the surface acoustic wave filter device according to the secondpreferred embodiment of the present invention, if a LiTaO₃ substratemade of a LiTaO₃ single crystal with an orientation rotated about the Xaxis from the Y axis to the Z axis within the range from about 36° toabout 44° is used, and if at least one electrode finger is inserted inat least one of the first and second spaces so that the electrodecovering ratio of the space in which the electrode finger is insertedbecomes equal to or greater than about 50%, propagation of leaky wavesbecomes dominant, and thus a reduction in the insertion loss isachieved. In particular, if the electrode covering ratio is equal to orgreater than about 63%, a further reduction in the insertion loss can beachieved.

In the surface acoustic wave filter device according to the secondpreferred embodiment of the present invention, if the distance betweenthe first reflector and the second reflector is substantially equal tothe distance between the third reflector and the fourth reflector, thefilter characteristics of the second and third surface acoustic wavefilter elements become substantially equal to each other, and thus, afurther suppression in the degradation in the degree of balance isachieved.

The surface acoustic wave filter device according to a third preferredembodiment includes the first, second and third surface acoustic wavefilter elements, wherein the second surface acoustic wave filter elementis connected to the second IDT of the first surface acoustic wave filterelement, the third surface acoustic wave filter element is connected tothe third IDT of the first surface acoustic wave filter element, andthere is a phase difference of about 180° within a passband between theinputs or the outputs of the second IDT and the third IDT of the firstsurface acoustic wave filter element. Thus, the surface acoustic wavefilter device can have the balance-unbalance conversion capability byusing the electrode connected to the first surface acoustic wave filterelement as an unbalanced terminal and using the terminals connected torespective second and third surface acoustic wave filter elements asbalanced terminals. In this surface acoustic wave filter deviceaccording to the first preferred embodiment of the present invention,unlike the conventional surface acoustic wave filter device in whichfour surface acoustic wave filter elements are used, thebalance-unbalance conversion capability is achieved using only threesurface acoustic wave filter elements. As a result, reductions in thesize and the cost of the surface acoustic wave filter device having thebalance-unbalance conversion capability are achieved.

Furthermore, the reduction in the number of surface acoustic wave filterelements results in a reduction in parasitic capacitance, whichsuppresses degradation in the degree of balance and which makes is easyto expand the passband.

In the surface acoustic wave filter device according to the thirdpreferred embodiment of the present invention, if the first space andthe second space are arranged such that they are different by an amountwithin the range from about 0.48λ to about 0.525λ, the amplitude balancebecomes equal to or lower than about 1.5 dB and the phase balancebecomes equal to or smaller than about 20°. That is, it is ensured thatdegradation in the degree of balance is prevented.

In the surface acoustic wave filter device according to the thirdpreferred embodiment of the present invention, if the first space andthe second space are arranged so as to satisfy Equations 1 and 2,respectively, a sufficiently large bandwidth can be obtained, anddegradation in the degree of balance is minimized.

Furthermore, in the third preferred embodiment of the present invention,if the first space and the second space are arranged so as to satisfyEquations 3 and 4, respectively, a sufficiently large bandwidth can beobtained and degradation in the degree of balance is minimized, evenwhen a variation in frequency due to a temperature variation is takeninto account.

If the first space is within the range from about 1.72λ to about 1.88λand the second space is set within the range from about 2.22λ to about2.33λ, the degradation in the degree of balance can be furthersuppressed and a sufficiently wide bandwidth can be achieved.

In the surface acoustic wave filter device according to the thirdpreferred embodiment of the present invention, if the distance betweenthe center of the first IDT and the first reflector and the distancebetween the center of the first IDT and the second reflector are set tobe substantially equal to each other, a further suppression in thedegradation in the degree of balance is achieved.

In the surface acoustic wave filter device according to one of the firstto third preferred embodiments, if the interdigital overlapping lengthof the electrode finger of the IDTs defining the first surface acousticwave filter element is within the range of about 1.5 to about 3.5 timesthe interdigital overlapping length of the electrode finger of the IDTsdefining the second and third surface acoustic wave filter element,degradation in VSWR in the passband is suppressed.

In the surface acoustic wave filter device according to a fourthpreferred embodiment of the present invention, the second surfaceacoustic wave filter element is arranged such that the transmissionamplitude characteristic of the second surface acoustic wave filterelement is substantially equal to that of the first surface acousticwave filter element and such that the transmission phase characteristicof the second surface acoustic wave filter element is different by about180° from that of the first surface acoustic wave filter element, andone end of each of the first and second surface acoustic wave filterelements is electrically connected in parallel, and the other end ofeach of the first and second surface acoustic wave filter elements iselectrically connected in series, so that the parallel-connectedterminals form unbalanced terminals and the series-connected terminalsdefine balanced terminals. Thus, a balance-unbalance conversioncapability is achieved, as in the surface acoustic wave filter deviceaccording to the first to third preferred embodiments. Furthermore,because only two surface acoustic wave filter elements are used, furtherreductions in the size and cost can be achieved.

In the surface acoustic wave filter element according to the fourthpreferred embodiment, if the first space and the second space arearranged such that they are different by an amount within the range fromabout 0.48λ to about 0.525λ, the amplitude balance becomes equal to orlower than about 1.5 dB and the phase balance becomes equal to orsmaller than about 20°. That is, it is ensured that degradation in thedegree of balance is prevented.

In the surface acoustic wave filter device according to the fourthpreferred embodiment of the present invention, if the first space andthe second space are arranged so as to satisfy Equations 1 and 2,respectively, a sufficiently large bandwidth can be obtained, anddegradation in the degree of balance is minimized.

Furthermore, if the first space and the second space are arranged so asto satisfy Equations 3 and 4, respectively, a sufficiently largebandwidth can be obtained and degradation in the degree of balance isminimized, even when a variation in frequency due to a temperaturevariation is taken into account.

In the fourth preferred embodiment, if the first space is within therange from about 1.72λ to about 1.88λ and the second space is within therange from about 2.22λ to about 2.33λ, the degradation in the degree ofbalance can be further suppressed and a sufficiently wide bandwidth canbe achieved.

Also in the fourth preferred embodiment of the present invention, if aLiTaO₃ substrate made of a LiTaO₃ single crystal with an orientationrotated about the X axis from the Y axis to the Z axis within the rangefrom about 36° to about 44° is used, and if at least one electrodefinger is inserted in at least one of the first and second spaces sothat the electrode covering ratio of the space in which the electrodefinger is inserted becomes equal to or greater than approximately 50%,propagation of leaky waves becomes dominant, and thus a reduction in theinsertion loss is achieved. In particular, if the electrode coveringratio is equal to or greater than about 63%, a further reduction in theinsertion loss can be achieved.

In the surface acoustic wave filter device according to the fourthpreferred embodiment of the present invention, if the distance betweenthe first reflector and the second reflector is substantially equal tothe distance between the third reflector and the fourth reflector, thefilter characteristics of the second and third surface acoustic wavefilter elements become substantially equal to each other, and thus afurther suppression in the degradation in the degree of balance isassured.

In the surface acoustic wave filter device according to the fourthpreferred embodiment of the present invention, if the unbalanced-sideterminal of the first surface acoustic wave filter element and theunbalanced-side terminal of the second surface acoustic wave filterelement are connected to each other via an electrode pattern on thepiezoelectric substrate, a reduction in parasitic capacitance isachieved, and thus a further reduction in the insertion loss isachieved.

In a fifth preferred embodiment of the present invention, the surfaceacoustic wave filter device includes first to third surface acousticwave filter elements, the first and second spaces are arranged so as tosatisfy Equation 1 and 2, respectively, the first IDT defines theunbalanced terminal, and the second and third IDTs are electricallyconnected in series so as to define the unbalanced terminal. Thus, thesurface acoustic wave filter device can have the balance-unbalanceconversion capability by using the electrode connected to the firstsurface acoustic wave filter element as an unbalanced terminal and usingthe terminals connected to respective second and third surface acousticwave filter elements as balanced terminals. In this surface acousticwave filter device according to the fifth preferred embodiment, unlikethe conventional surface acoustic wave filter device in which foursurface acoustic wave filter elements are used, the balance-unbalanceconversion capability is achieved using only three surface acoustic wavefilter elements. As a result, reductions in the size and the cost of thesurface acoustic wave filter device having the balance-unbalanceconversion capability are achieved.

Furthermore, the reduction in the number of surface acoustic wave filterelements results in a reduction in parasitic capacitance, whichsuppresses degradation in the degree of balance and which makes it easyto expand the passband.

Similarly, also in the surface acoustic wave filter device according toa sixth or seventh preferred embodiment, the surface acoustic wavefilter device can have the balance-unbalance conversion capability byusing the electrode connected to the first surface acoustic wave filterelement as an unbalanced terminal and using the terminals connected torespective second and third surface acoustic wave filter elements asbalanced terminals. In this surface acoustic wave filter deviceaccording to the first through sixth preferred embodiments, unlike theconventional surface acoustic wave filter device in which four surfaceacoustic wave filter elements are used, the balance-unbalance conversioncapability is achieved using only three surface acoustic wave filterelements. As a result, reductions in the size and the cost of thesurface acoustic wave filter device having the balance-unbalanceconversion capability are achieved.

Furthermore, the reduction in the number of surface acoustic wave filterelements results in a reduction in parasitic capacitance, whichminimizes degradation in the degree of balance and which makes is easyto expand the passband.

The surface acoustic wave filter device according to an eighth preferredembodiment also has the balance-unbalance conversion capability as inthe first preferred embodiment. Furthermore, because the particular typeof piezoelectric substrate is used as the piezoelectric substrate,propagation of leaky waves becomes dominant, and thus a reduction in theinsertion loss is achieved. In particular, if the electrode coveringratio is equal to or greater than about 63%, a further reduction in theinsertion loss is achieved. Furthermore, the first and second spaces arearranged so as to be different from each other by about 0.48λ to about0.525λ so that the amplitude balance equal to or less than about 1.5 dBand the phase balance equal to or smaller than about 20° can beachieved. Thus, degradation in the degree of balance is prevented. Atleast one electrode finger is disposed in each area between an electrodefinger, which is one of the first IDT's electrode fingers connected to asignal line and which is disposed at an outermost location, and anelectrode finger which is one of the second or third IDT's electrodefingers connected to a signal line and which is disposed at a locationclosest to the center, so that the electrode covering ratio in each areadescried above becomes equal to or greater than about 50% therebyachieving a further reduction in the insertion loss.

In particular, if the electrode covering ratio is equal to or greaterthan about 63%, a further reduction in the insertion loss can beachieved.

In the present preferred embodiment, when a series terminal is connectedon the unbalanced terminal side, the characteristic in terms ofattenuation outside the passband is greatly improved.

Similarly, if surface acoustic wave resonators are connected in seriesto the respective terminals on the balanced terminal side, thecharacteristic in terms of attenuation outside the passband is greatlyimproved.

In the case where a surface acoustic wave filter in the form of acascaded ladder circuit is disposed on the balanced terminal side, ifthe ladder-type surface acoustic wave filter is arranged such that itsattenuation poles are located near the lower and upper edges of thepassband, greater attenuation and better selectivity can be achieved.Furthermore, the surface acoustic wave resonator connected in parallelbetween the balanced terminals imposes equal influences upon bothbalanced terminals. This makes it possible to increase the attenuationoutside the passband without causing degradation in the degree ofbalance.

In the surface acoustic wave filter device according to the presentpreferred embodiment, in the case where a chip on which surface acousticwave filter elements are disposed is housed in a package, if at leastone of the electrode pattern, the package, and the electrical connectionmember has a structure which are substantially axially symmetric,further suppression in degradation in the degree of balance can beachieved.

In particular, when at least two of the electrode pattern, the package,and the electrical connection member have structures which aresubstantially axially symmetric with respect to the same symmetry axis,further suppression in degradation in the degree of balance can beachieved.

The surface acoustic wave filter according to various preferredembodiments of the present embodiment can be used in duplexers orcommunication devices, as described above. In this case, a reduction inthe size of the duplexers and communication devices can be achieved.

For the purpose of illustrating the invention, there is shown in thedrawings several forms which are presently preferred, it beingunderstood, however, that the present invention is not limited to theprecise arrangements and instrumentalities shown.

Other features, elements, characteristics and advantages of the presentinvention will become apparent from the detailed description ofpreferred embodiments thereof with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view schematically illustrating the electrode structureof a surface acoustic wave filter device according to a first preferredembodiment of the present invention.

FIG. 2 is graph illustrating the filter characteristic of the surfaceacoustic wave filter device according to the first preferred embodimentof the present invention and also illustrates the filter characteristicof a conventional surface acoustic wave filter device.

FIG. 3 is a graph illustrating VSWR at an unbalanced terminal for boththe surface acoustic wave filter device according to the first preferredembodiment of the present invention and the conventional surfaceacoustic wave filter device.

FIG. 4 is a graph illustrating VSWR at balanced terminals for both thesurface acoustic wave filter device according to the first preferredembodiment of the present invention and the conventional surfaceacoustic wave filter device.

FIG. 5 is a graph illustrating the dependence of the amplitude balanceupon the space between adjacent IDTs.

FIG. 6 is a graph illustrating the dependence of the phase balance uponthe space between adjacent IDTs.

FIG. 7 is a graph illustrating the dependence of the bandwidth upon thespace between adjacent IDTs.

FIG. 8 is a graph illustrating the dependence of the insertion losswithin the passband upon the space between adjacent IDTs.

FIG. 9 is a graph illustrating the dependence of the amplitude balanceupon the space between adjacent IDTs.

FIG. 10 is a graph illustrating the dependence of the phase balance uponthe space between adjacent IDTs.

FIG. 11 is a graph illustrating the dependence of the 4.0-dB bandwidthupon the ratio of the interdigital overlapping length of the electrodefinger.

FIG. 12 is a graph illustrating the dependence of VSWR upon the ratio ofthe interdigital overlapping length of the electrode finger.

FIG. 13 is a plan view schematically illustrating the electrodestructure of a surface acoustic wave filter device according to a secondpreferred embodiment of the present invention.

FIG. 14 is a plan view schematically illustrating the electrodestructure of a surface acoustic wave filter device according to a thirdpreferred embodiment of the present invention.

FIG. 15 is a plan view schematically illustrating the electrodestructure of a surface acoustic wave filter device according to a fourthpreferred embodiment of the present invention.

FIG. 16 is a graph illustrating the filter characteristic of the surfaceacoustic wave filter device according to the fourth preferred embodimentof the present invention.

FIG. 17 is a plan view schematically illustrating the electrodestructure of a surface acoustic wave filter device according to a fifthpreferred embodiment of the present invention.

FIG. 18 is a plan view schematically illustrating the electrodestructure of a surface acoustic wave filter device according to a sixthpreferred embodiment of the present invention.

FIG. 19 is a plan view schematically illustrating the electrodestructure of a surface acoustic wave filter device according to aseventh preferred embodiment of the present invention.

FIG. 20 is a plan view schematically illustrating the electrodestructure of a surface acoustic wave filter device according to aneighth preferred embodiment of the present invention.

FIG. 21 is a graph illustrating the filter characteristic for both thesurface acoustic wave filter device according to the fourth preferredembodiment and that according to the eighth preferred embodiment of thepresent invention.

FIG. 22 is a plan view schematically illustrating the electrodestructure of a surface acoustic wave filter device according to a ninthpreferred embodiment of the present invention.

FIG. 23 is a plan view schematically illustrating the electrodestructure of a surface acoustic wave filter device according to a tenthpreferred embodiment of the present invention.

FIG. 24A is a perspective exploded view illustrating a surface acousticwave filter device according to an eleventh preferred embodiment of thepresent invention.

FIGS. 24B and 24C are plan views each showing a bottom surface ofpackage in which surface acoustic wave filter according to variouspreferred embodiments of the present invention is provided.

FIG. 25 is a plan view illustrating a surface acoustic wave filterdevice according to a twelfth preferred embodiment of the presentinvention.

FIG. 26 is a schematic diagram illustrating an antenna duplexerconstructed to incorporate a surface acoustic wave filter deviceaccording to the twelfth preferred embodiment of the present invention.

FIG. 27 is a schematic diagram illustrating a modification of thesurface acoustic wave filter device according to the twelfth preferredembodiment of the present invention.

FIG. 28 is a plan view schematically illustrating an example of aconventional surface acoustic wave filter device.

FIG. 29 is a plan view schematically illustrating another example of aconventional surface acoustic wave filter device.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention is described in further detail below withreference to specific preferred embodiments of surface acoustic wavefilter devices in conjunction with the drawings.

Referring to FIG. 1, a first preferred embodiment of a surface acousticwave filter device according to the present invention is describedbelow.

FIG. 1 is a plan view illustrating the electrode structure of the firstpreferred embodiment of the surface acoustic wave filter device. In thepresent preferred embodiment of the surface acoustic wave filter device,three surface acoustic wave filter elements 1 to 3 are disposed on apiezoelectric substrate. A substrate made of a proper piezoelectricmaterial such as LiTaO₃ or quartz is preferably used as thepiezoelectric substrate. In this preferred embodiment, a 36° Y-X LiTaO₃substrate is preferably used.

The first surface acoustic wave filter element 1 has three IDTs 1 a to 1c disposed along the surface wave propagation direction. In the areawhere the IDTs 1 a to 1 c are disposed, grating-type reflectors 1 d and1 e are disposed at respective ends in the surface wave propagationdirection.

Similarly, the second and third surface acoustic wave filter elements 2and 3 have a structure in which three IDTs 2 a to 2 c or 3 a to 3 c arearranged in the surface wave propagation direction. Also, in the secondand third surface acoustic wave filter elements 2 and 3, grating-typereflectors 2 d, 2 e, 3 d, and 3 e are disposed at respective ends, inthe surface wave propagation direction, outside the areas where the IDTs2 a to 2 c and 3 a to 3 c are disposed. Each IDT 1 a to 1 c, 2 a to 2 c,and 3 a to 3 c has a pair of comb-shaped electrodes.

One comb-shaped electrode of the central IDT 1 a of the first surfaceacoustic wave filter element 1 is connected to an input terminal 4. Onecomb-shaped electrode of each of the second and third IDTs 1 b locatedoutside of the first IDT 1 a at the central location is electricallyconnected to one of the comb-shaped electrodes of each of the second andthird IDTs 2 b and 2 c disposed at outer locations in the second surfaceacoustic wave filter element. Similarly, one of comb-shaped electrodesof the IDT 1 c at an outer location in the first surface acoustic wavefilter element is electrically connected to one of comb-shapedelectrodes of each of the IDTs 3 b and 3 c at outer locations in thethird surface acoustic wave filter element 3. One of comb-shapedelectrodes of each IDTs 2 a and 3 a at the central location in each ofthe second and third surface acoustic wave filter elements iselectrically connected to an output terminal 5 or 6. The othercomb-shaped electrode of each of the IDTs 1 a to 1 c, 2 a to 2 c, and 3a to 3 c is grounded.

The input terminal 4 is an unbalanced terminal, and the output terminals5 and 6 are balanced terminals.

The transmission phase characteristic of the third surface acoustic wavefilter element 103 is different by about 180° from the transmissionphase characteristic of the second surface acoustic wave filter element102.

Specific examples of the structures of the first to third surfaceacoustic wave filter elements 1 to 3 are described below.

In the present preferred embodiment, IDTs 1 a to 1 c in the firstsurface acoustic wave filter element 1 have an interdigital overlappinglength W of the electrode finger substantially equal to 52λ where λ isthe wavelength of the surface acoustic wave. The first IDT 1 a disposedat the central location has 16 pairs of electrode fingers, and the IDTsdisposed at outer locations, that is, the second and third IDTs 1 b and1 c each have 11 pairs of electrode fingers. In the IDTs 1 a to 1 c, thewavelength λI of the surface acoustic wave is substantially equal toabout 4.2 μm. The reflectors 1 d and 1 e each have 120 electrodefingers, and the wavelength λR is substantially equal to about 4.3 μm.The space GI between adjacent IDTs 1 a to 1 c is substantially equal toabout 1.77λR. Herein, the space between adjacent IDTs is defined asfollows. For example, the space between IDTs 1 a and 1 b is defined asthe pitch between hot electrode fingers which belong to the respectiveIDTs 1 a and 1 b and which are closest to each other.

In the second surface acoustic wave filter element 2, the interdigitallyoverlapping lengths W of the electrode finger of the IDTs 2 a to 2 c aresubstantially equal to about 31λ. The first IDT 2 a disposed at theapproximately central location has 16 pairs of electrode fingers, andthe IDTs disposed at outer locations, that is, the second and third IDTs2 b and 2 c each have 11 pairs of electrode fingers. In the IDTs 2 a to2 c, the wavelength λI of the surface acoustic wave is substantiallyequal to about 4.2 μm. The reflectors 2 d and 2 e each have 120electrode fingers, and the wavelength λ is substantially equal to about4.3 μm. The space GI between adjacent IDTs 2 a to 2 c is substantiallyequal to about 1.77λR.

The third surface acoustic wave filter element 103 is constructed in thesame manner as the second surface acoustic wave filter element 2 exceptthat the space GI between adjacent IDTs is substantially equal to about2.27 λR.

The space GI between adjacent IDTs is preferably different for thesecond surface acoustic wave filter element 2 and the third surfaceacoustic wave filter element 3 so that the transmission phasecharacteristic becomes different by about 180°. Note that the manner ofcreating a difference of 180° in the transmission phase characteristicbetween the second and third surface acoustic wave filter elements 2 and3 is not limited to setting the space between adjacent IDTs to differentvalues.

In the present preferred embodiment, and also in the following preferredembodiments of the present invention, the surface acoustic wave filterelements and the reflectors have a great number of electrode fingers,and thus they are represented in simplified fashions in the figures.

The operation of the surface wave acoustic filter element of the presentpreferred embodiment is described below for the case where the inputterminal 4 is used as an unbalanced input terminal and the outputterminals 5 and 6 are used as balanced output terminals.

When an electrical signal is applied to the input terminal 4, theelectrical signal is filtered by the first surface acoustic wave filterelement 1, and the resultant electrical signal is applied to the secondand third surface acoustic wave filter elements 2 and 3. Herein, if theIDTs 1 b and 1 c in the surface acoustic wave filter element 1 have thesame structure and if the distance from the IDT 1 a to the IDT 1 b andthat from the IDT 1 a to the IDT 1 c are substantially equal to eachother, the same electrical signal is applied to the second and thirdsurface acoustic wave filter elements 2 and 3.

The electrical signals applied to the surface acoustic wave filterelement 2 and that applied to the surface acoustic wave filter element 3are again filtered and supplied to the balanced output terminals 5 and6.

As described above, the surface acoustic wave filter element 2 and thesurface acoustic wave filter element 3 are constructed in the samemanner except for the difference in the space GI between adjacent IDTs.Therefore, in the filtering, the surface acoustic wave filter element 2and the surface acoustic wave filter element 3 are the same in theamplitude characteristic but are different by about 180° in thetransmission phase characteristic. As a result, the electrical signalssupplied to the output terminals 5 and 6 are the same in the amplitudecharacteristic but are different by about 180° in the transmission phasecharacteristic, and thus the electrical signals supplied to the outputterminals 5 and 6 become perfectly balanced signals.

The parameters such as an interdigital overlapping length of the surfaceacoustic wave filter element 1 are set such that the surface acousticwave filter element 1 matches the characteristic impedance, for example,50 Ω, of an unbalanced circuit such as an antenna circuit connected tothe input terminal 4. The parameters such as an interdigital overlappinglength of the surface acoustic wave filter elements 2 and 3 are set soas to match one-half the characteristic impedance of 150 Ω of anbalanced circuit such as an amplifier connected to the output terminals5 and 6. Note that when each of the terminals 5 and 6 of the balancedcircuit is regarded as an unbalanced terminal, the characteristicimpedance becomes substantially equal to one-half the characteristicimpedance of the balanced circuit.

In the surface acoustic wave filter device having thebalance-to-unbalance conversion capability disclosed in JapaneseUnexamined Patent Application Publication No. 10-117123 cited above, twosurface acoustic wave filter elements are required to be disposed on theinput side (unbalanced side).

In contrast, in the present preferred embodiment, only one surfaceacoustic wave filter element is required to be disposed on the inputside (unbalance side) as described above. Therefore, compared with theprior art described above, the present preferred embodiment allows greatreductions in the parasitic capacitance between bus bars of adjacentIDTs, the parasitic capacitance associated with interconnection linesbetween the surface acoustic wave filter element on the input side andsurface acoustic wave filter elements on the output side, the parasiticcapacitance associated with electrodes extending on a chip betweenpackage electrodes and the surface acoustic wave filter device, and theparasitic capacitance associated with bonding pads. The parasiticcapacitances such as those described above are major factors which makeit difficult to increase the bandwidth of surface acoustic wave filterdevices.

In the surface acoustic wave filter device according to the presentpreferred embodiment, it is possible to reduce the parasitic resistancesas described above, and thus it is possible to achieve a wideband filtercharacteristic without causing degradation in the flatness or VSWR inthe passband.

The filter characteristic obtained in the present preferred embodimentis represented by a solid line in FIG. 2. For the purpose of comparison,the filter characteristic of a surface acoustic wave filter device,which has been produced in accordance with the description in JapaneseUnexamined Patent Application Publication No. 10-117123 so as to have asimilar passband to that of the surface acoustic wave filter deviceaccording to the present preferred embodiment, is represented by abroken line.

As can be seen from FIG. 2, the present preferred embodiment allows thesurface acoustic wave filter device to have a wideband filtercharacteristic.

FIGS. 3 and 4 show the VSWR characteristic at the unbalanced andbalanced terminals of the surface acoustic wave filter devices producedin accordance with the present preferred embodiment and the prior artdescribed above. In these figures, solid lines represent thecharacteristic of the surface acoustic wave filter device according tothe present preferred embodiment, and the broken lines represent thecharacteristic of the surface acoustic wave filter device according tothe prior art. As can be seen from FIGS. 3 and 4, the present preferredembodiment greatly suppresses degradation in VSWR.

Furthermore, the surface acoustic wave filter device according to thepresent preferred embodiment needs only three surface acoustic wavefilter elements, and thus it is possible to reduce the chip size. Thisallows a reduction in the total size of the surface acoustic wave filterdevice and makes it possible to obtain a greater number of surfaceacoustic wave filter devices from each wafer in production of surfaceacoustic wave filter devices, and thus a reduction in cost can beachieved.

FIGS. 5 and 6 show the degree of balance as a function of thedifference, in terms of the space between adjacent IDTs, between thesurface acoustic wave filter elements 2 and 3 for the case where thespace GI between the adjacent IDTs of the second surface acoustic wavefilter element is preferably fixed at about 1.77λ, and the space GIbetween the adjacent IDTs of the third surface acoustic wave filterelement 3 is varied. Herein, the space between adjacent IDTs is definedas the distance from the center of an electrode finger, which is one ofone IDT's electrode fingers, not grounded but connected to a signalline, and which is closest to the other IDT, to the center of anelectrode finger which is one of the other IDT's electrode fingers, notgrounded but connected to a signal line, and which is closest to the oneIDT. Note that the space differences taken along the horizontal axes inFIGS. 5 and 6 are normalized to λ.

Herein, the amplitude balance and the phase balance are defined asfollows. When the surface acoustic wave filter device of the presentpreferred embodiment is regarded as a 3-port device in which theunbalanced input terminal functions as a port 1 and the balanced outputterminals 5 and 6 function as ports 2 and 3, respectively, the amplitudebalance |A| is given by A=|S21|−|S31|, and the phase balance |B-180| isgiven by B=|<S21−<S31|.

Ideally, the amplitude balance is 0 dB and the phase balance is 0°.However, the amplitude balance less than about 1.5 dB and the phasebalance less than about 20° are allowed in practical use.

It can be seen from FIG. 5 that the amplitude balance falls within theallowable range if the IDT space difference between the surface acousticwave filter elements 2 and 3 is smaller than about 0.525λ. On the otherhand, FIG. 6 indicates that the phase balance falls within the allowablerange if the IDT space difference between the surface acoustic wavefilter elements 2 and 3 is in the range from about 0.48λ to about0.525λ. Therefore, both the amplitude balance and the phase balance fallwithin the respective allowable ranges if the difference between the IDTspace of the surface acoustic wave filter element 2 and the IDT space ofthe surface acoustic wave filter element 3 is within the range fromabout 0.48λ to about 0.525λ.

In the case of a surface acoustic wave filter element of thecascade-coupled resonator type having three IDTs, it is known to achievea wideband filter characteristic by setting the space between adjacentIDTs within the range from (0.72+n/2)×λ to (0.83+n/2)×λ, where n=0, 1,2, . . . , 6. This means that there can be various different rangeswithin which the difference in IDT space between the surface acousticwave filter element 2 and the surface acoustic wave filter element 3should be taken.

However, if the value of n in the above formula is too great, thefollowing problem occurs. FIG. 7 shows the dependence of the bandwidthof the surface acoustic wave filter device upon the space betweenadjacent IDTs for the case where the space between adjacent IDTs of thesurface acoustic wave filter element 2 of the surface acoustic wavefilter device according to the present preferred embodiment issubstantially equal to (n/2+0.77)×λ where n=0, 1, 2, . . . , 6. As canbe seen from FIG. 7, n should be equal to or smaller than 6 to meet therequirement that the bandwidth of surface acoustic wave filter devicesfor use in portable telephones should be equal to or greater than about35 MHz.

On the other hand, if n is too small, another problem occurs. FIG. 9shows the dependence of the amplitude balance of the surface acousticwave filter device of the present preferred embodiment upon the space GIbetween adjacent IDTs in the second surface acoustic wave filter element2 for the case where the space GI between adjacent IDTs of the surfaceacoustic wave filter element 2 is substantially equal to (0.77+m/2)×λand the space GI between adjacent IDTs of the surface acoustic wavefilter element 3 is substantially equal to (1.27+m/2)×λ where m=0, 1, 2,. . . .

FIG. 10 shows the dependence of the phase balance of the surfaceacoustic wave filter device of the present preferred embodiment upon thespaces GI between adjacent IDTs in the second surface acoustic wavefilter element 2 for the case where the space GI between adjacent IDTsof the surface acoustic wave filter element 2 is substantially equal to(0.77+m/2)×λ and the space GI between adjacent IDTs of the surfaceacoustic wave filter element 3 is substantially equal to (1.27+m/2)×λ.

From FIGS. 9 and 10, it can be seen that in order to meet therequirement that the amplitude balance should be equal to or less thanabout 1.5 dB and the phase balance should be equal to or less than about10°, the space between IDTs should be equal to or greater than about1.77λ and m should be equal to or greater than 1. The reason why thebalance becomes worse when the space between adjacent IDTs is small canbe understood as follows.

In the case of surface acoustic wave filter devices of thecascade-coupled resonator type, adjacent IDTs are coupled not onlyacoustically but also can be coupled electromagnetically. In thetransmission characteristic resulting from the acoustic coupling, thephases of the surface acoustic wave filter elements 2 and 3 becomeopposite to each other, if the space between adjacent IDTs is differentby about 0.5λ between the surface acoustic wave filter elements 2 and 3.However, the transmission characteristic resulting from theelectromagnetic coupling does not depend upon the space between adjacentIDTs, and thus an equal phase and an equal amplitude are obtained. Thetransmission components having the same phase and the same amplitudecause degradation in the degree of balance. Thus, a reduction in thespace between IDTs results in an increase in the electromagneticcoupling which in turn results in degradation in the degree of balance.

From the above discussion, it can be concluded that the filtercharacteristic in terms of the degree of balance and the bandwidth canbe good enough for practical use, if the spaces A₁ and A₂ betweenadjacent IDTs in the surface acoustic wave filter element 2 aresubstantially equal to (0.77+n/2)×λ where n=1, 2, 3, 4, 5 and if thespace GI between adjacent IDTs in the surface acoustic wave filterelement 3 is substantially equal to (1.27+n/2)×λ where n is an integerfrom 1 to 5.

When the frequency variation due to a temperature variation is takenaccount, the bandwidth is required to be equal to or greater than about39 MHz. This requirement can be met if the spaces A₁ and A₂ betweenadjacent IDTs in the surface acoustic wave filter element 2 aresubstantially equal to (0.77+n/2)×λ where n is an integer from 1 to 3and if the space GI between adjacent IDTs in the surface acoustic wavefilter element 3 is substantially equal to (1.27+n/2)×λ where n is aninteger from 1 to 3.

If the space GI between adjacent IDTs in the surface acoustic wavefilter element 2 is substantially equal to (0.77+n/2)×λ where n=2 and ifthe spaces B₁ and B₂ between adjacent IDTs in the surface acoustic wavefilter element 3 are substantially equal to (1.27+n/2)×λ where n=2, thelargest bandwidth can be achieved without causing degradation in thedegree of balance.

When the piezoelectric substrate is made of a LiTaO₃ single crystal withan orientation rotated around the X axis by about 36° to about 44° fromthe Y axis to the Z axis, two types of surface acoustic waves can beexcited and propagated. One is a leaky wave or a pseudo surface acousticwave, and the other is a bulk wave called SSBW. Of these, the leaky waveis mainly used in resonators or filters. If propagation of SSBW becomesdominant, the propagation loss becomes large. As a result, degradationin Q occurs in resonators and the insertion loss of filters becomeslarge. The two types of surface acoustic waves described above areexcited and propagated in a mixed fashion. When the surface is in analmost electrically short-circuited state, that is, when the electrodecovering ratio is large, propagation of leaky waves becomes dominant.Conversely, when the surface is in an almost electrically open state,that is, when the electrode covering ration is small, propagation ofSSBW becomes dominant.

Therefore, if, in the second surface acoustic wave filter element, atleast one electrode finger is disposed in first spaces between the IDTat the central location and the second and third IDTs at outerlocations, and if, in the third surface acoustic wave filter element, atleast one electrode finger is disposed in second spaces between the IDTat the central location and the second and third IDTs at outer locationsthereby increasing the electrode covering ratio, propagation of leakywaves becomes dominant and excitation and propagation of SSBW aresuppressed and thus a reduction in the insertion loss is achieved.

FIG. 8 illustrates the dependence of the insertion loss in the passbandupon the electrode covering ratio in the first spaces described above.It can be seen that the electrode covering ratio should be equal to orgreater than about 0.5 or about 50% to obtain a low insertion loss equalto or smaller than about 3.0 dB within the passband, which is requiredin practical use. In order to obtain an insertion loss lower than about2.5 dB for use in applications in which low loss is required, theelectrode covering ratio should be equal to or greater than about 0.63or about 63%. The above discussion applies to the second spaces.

The signal input to the second surface acoustic wave filter element 2excites surface acoustic waves via the IDTs 2 b ad 2 c. The surfaceacoustic waves propagate in the particular propagation directions andare reflected by the reflectors 2 d and 2 e. The reflected surfaceacoustic waves interfere with the excited surface acoustic waves. As aresult, a standing wave is created between the reflectors 2 d and 2 e.The standing wave allows the resonance to have very high Q. Furthermore,the excited standing wave is received by the IDT 2 a and converted intoan electrical signal by the IDT 2 a and thus a function of a filter isachieved. A similar operation is also performed in the third surfaceacoustic wave filter element 3. However, in the third surface acousticwave filter element 3, because the output signal is determined by therelative positional relationship between the excited standing wave andthe IDT 3 a at the output side, the location of the IDT 3 a is shiftedby about 0.5 times the wavelength λ of the surface acoustic wave so thatthe output signal has a phase opposite to the output signal of thesecond surface acoustic wave filter element.

Herein, if the distance C between the two reflectors 2 d and 2 e of thesurface acoustic wave filter element 2 and the distance D between thetwo reflectors 3 d and 3 e of the surface acoustic wave filter element 3are different from each other, the amplitude distribution becomesdifferent between the two surface acoustic wave filter elements. As aresult, the resonance characteristic and the filter characteristic alsobecome different. In view of the above, the distance C between the tworeflectors 2 d and 2 e of the surface acoustic wave filter element 2 andthe distance D between the two reflectors 3 d and 3 e of the surfaceacoustic wave filter element 3 are preferably substantially equal toeach other so that no difference occurs in the filter characteristicbetween the surface acoustic wave filter elements 2 and 3 and thus nodegradation occurs in the degree of balance.

In the present preferred embodiment, the grating type reflectors areused as the reflectors 1 d, 1 e, 2 d, 2 e, 3 d, and 3 e. However, thereflectors are not limited to the grating type. For example, reflectorsusing reflection at the ends of the piezoelectric substrate may also beused.

In the present preferred embodiment, the characteristic impedance at theinput terminal (unbalanced terminal) 4 is preferably about 50 Ω and thecharacteristic impedance at the output terminals 5 and 6 (balancedterminals) is preferably about 150 Ω. That is, in the present preferredembodiment, as described above, the interdigitally overlapping length ofthe surface acoustic wave filter element 1 is preferably equal to about51λ so that the input impedance matches the characteristic impedance ofabout 50 Ω of an unbalanced circuit connected to the input terminal. Onthe other hand, in the surface acoustic wave filter element 2 and 3, theinterdigitally overlapping length is preferably about 31λ so that theoutput impedance matches one-half the characteristic impedance of 150 Ωof a balanced circuit connected to the output terminal, taking intoaccount the fact that when each of the terminals 5 and 6 of the balancedcircuit is regarded as an unbalanced terminal, the characteristicimpedance becomes equal to one-half the characteristic impedance of thebalanced circuit.

The ratio of the output impedance to the input impedance can be set toan arbitrary desired value by achieving the impedance matching with theunbalanced circuit connected to the input terminal, using the surfaceacoustic wave filter element 1, and achieving the impedance matchingwith the balanced circuit connected to the output terminals, using thesurface acoustic wave filter elements 2 and 3.

FIG. 11 shows the dependence of the bandwidth upon the ratio of theinterdigital overlapping length of the surface acoustic wave filterelement 1 connected to the unbalanced terminal 4 to the interdigitaloverlapping length of the surface acoustic wave filter elements 2 and 3connected to the balanced terminals. It can be seen from FIG. 11 thatthe bandwidth becomes maximum when the interdigital overlapping lengthratio is substantially equal to about 2.0. If the interdigitaloverlapping length ratio is greater than about 3.5, the bandwidthdecreases by 5% or greater, and the yield decreases.

FIG. 12 shows the dependence of VSWR within the passband upon the ratioof the interdigitally overlapping length of the surface acoustic wavefilter element 1 connected to the unbalanced terminals 5 and 6 to theinterdigitally overlapping length of the surface acoustic wave filterelements 2 and 3 connected to the balanced terminals. VSWR becomes bestwhen the interdigitally overlapping length ratio is substantially equalto about 2.5. If the interdigitally overlapping length ratio is smallerthan about 1.5, VSWR becomes very bad and problems occur in practicaluse. Thus, it is desirable to set the ratio of the interdigitallyoverlapping length of electrode fingers within the range from about 1.5to about 3.5.

FIG. 13 is a plan view illustrating the electrode structure of a secondpreferred embodiment of a surface acoustic wave filter device accordingto the present invention. In the present preferred embodiment, threesurface acoustic wave filter elements 11 to 13 are disposed on apiezoelectric substrate that is not shown in the figure. A substratemade of a proper piezoelectric material such as LiTaO₃ or quartz may beused as the piezoelectric substrate. In this preferred embodiment, a 36°Y-X LiTaO₃ substrate is preferably used. The basic structure and thestructure associated with connections of the first to third surfaceacoustic wave filter elements 1 to 13 are similar to those in the firstpreferred embodiment, and similar elements are denoted by similarreference numerals and they are not described further herein.

The surface acoustic wave filter device of the second preferredembodiment is different in the electrode structure of the first to thirdsurface acoustic wave filter elements 11 to 13 from the surface acousticwave filter device of the first preferred embodiment.

In the present preferred embodiment, as will be described later, thesurface acoustic wave filter elements 11 and 12 are constructed suchthat the electrical signals output from the IDTs disposed at outerlocations in the first surface acoustic wave filter element 11, that is,the second and third IDTs 11 b and 11 c, are different in transmissionphase characteristic by about 180°. Thus, the electrical signals whichare the same in amplitude but different in phase by about 180° areapplied to the second and third surface acoustic wave filter elements 12and 13.

In the first surface acoustic wave filter element 11 of the presentpreferred embodiment, the interdigital overlapping length W of theelectrode finger of the IDTs 11 a to 11 c is preferably equal to about52λ, where λ is the wavelength of the surface acoustic wave.

In the first surface acoustic wave filter element 11, the first IDT 1 adisposed at the central location has 16 pairs of electrode fingers, andthe IDTs disposed at outer locations, that is, the second and third IDTs11 b and 11 c each have 11 pairs of electrode fingers. In the IDTs 11 ato 11 c, the wavelength λI of the surface acoustic wave is preferablyequal to about 4.2 μm. The reflectors 11 d and 11 e each have 120electrode fingers, and the wavelength λR is preferably equal to about4.3 μm. The space A₁ between the first IDT 11 a and the second IDT 11 bis preferably equal to about 1.77λR, and the space B₁ between the firstIDT 11 a and the third IDT 11 c is preferably equal to about 2.27λR.

In the second surface acoustic wave filter element 12, the interdigitaloverlapping length W of the electrode finger is preferably equal toabout 31λ, the first IDT 12 a disposed at the central location has 16pairs of electrode fingers, and the IDTs at outer locations, that is,the second and third IDTs 12 b and 12 c each have 11 pairs of electrodefingers. In the IDTs 12 a to 12 c, the wavelength λI of the surfaceacoustic wave is preferably equal to about 4.2 μm. The reflectors 12 dand 12 e each have 120 electrode fingers, and the wavelength λR ispreferably equal to about 4.3 μm. The space A₂ between the first IDT 12a and the second IDT 12 b is preferably equal to about 1.77λR, and thespace B₂ between the first IDT 12 a and the third IDT 12 c is preferablyequal to about 1.77λR.

The third surface acoustic wave filter element 13 is preferablyconstructed in the same manner as the second surface acoustic wavefilter element 13. The operation of the surface wave acoustic filterelement of the second preferred embodiment is described below for thecase where the input terminal 4 is used as an unbalanced input terminaland the output terminals 5 and 6 are used as balanced output terminals.

When an electrical signal is applied to the input terminal 4, a surfaceacoustic wave is excited by the first IDT 11 a of the first surfaceacoustic wave filter element. The surface acoustic wave propagates in adirection that is substantially perpendicular to the direction in whichthe electrode fingers extend and is reflected by reflectors 11 d and 11e. The reflected surface acoustic wave interferes with the excitedsurface acoustic wave. As a result, a standing wave is created betweenthe two reflectors 11 d and 11 e. The standing wave allows resonance tooccur with very high Q. The excited standing wave is received by theIDTs 11 b and 11 c located on the output side and converted toelectrical signals. Thus, the first surface acoustic wave filter element11 operates as a filter.

Herein, the output signal is determined by the relative positionalrelationship between the standing wave and the IDTs 11 b and 11 clocated on the output side, the location of either the IDT 11 b or theIDT 11 c is shifted by about 0.5 times the wavelength λ of the surfaceacoustic wave so that the phase is inverted. In the second preferredembodiment, the first and second spaces A₁ and B₁ are determined asdescribed above so that the electrical signal output from the IDT 11 band the electrical signal output from the IDT 11 c are different inphase by about 180° from each other. As a result, the electrical signalswhich are the same in amplitude but different in phase by about 180° areapplied to the second and third surface acoustic wave filter elements 12and 13. The output signals are filtered by the second and third surfaceacoustic wave filter elements 12 and 13, and the resultant signals areoutput as balanced signals to the output terminals 5 and 6.

The results shown in FIGS. 5 and 6 also apply to this second preferredembodiment, and thus the difference between the first space A₁ betweenthe IDT 1 a and the IDT 1 b and the second space B₁ between the IDT 1 aand the IDT 1 c is preferably set within the range from about 0.48λ toabout 0.525λ.

A wideband characteristic can be achieved without having degradation inthe degree of balance, by setting the above-described IDT-to-IDT spacesto be within a combination of the range from about (n/2+1.22)×λ to about(n/2+1.33)×λ (n is an integer of 0 to 4) and the range from about(n/2+1.72)×λ to about (n/2+1.83)×λ (n is an integer of 0 to 4).

As in the first preferred embodiment, if, in the second surface acousticwave filter element 12, at least one electrode finger is disposed infirst spaces between the IDT 12 a at the central location and the secondand third IDTs 12 b and 12 c at outer locations, and if, in the thirdsurface acoustic wave filter element, at least one electrode finger isdisposed in second spaces between the first IDT 13 a at the centrallocation and the second and third IDTs 13 b and 13 c at outer locationsthereby increasing the electrode covering ratio, propagation of leakywaves becomes dominant and thus a reduction in the insertion loss isachieved. In the present preferred embodiment, in view of the above, theelectrode covering ratio in the first and second spaces described aboveis preferably about 63% to achieve a reduction in the insertion loss.

In the present preferred embodiment, the first and second spaces arepreferably different from each other so that degradation in theamplitude balance is prevented.

Furthermore, in the first surface acoustic wave filter element 11, thedistance P between the first IDT 11 a the reflector 11 d and thedistance P₂ between the first IDT 11 a and the reflector 11 e arepreferably substantially equal to each other so that the excitedamplitude distribution of the standing wave created in the first surfaceacoustic wave filter element does not become asymmetrical. As a result,the intensities of the surface acoustic waves received by the IDTs 11 ban 11 c become substantially equal, and suppression in degradation inthe degree of balance is achieved. Herein, the distance P is defined asthe distance from the center of an electrode finger, which is one of theIDT 11 a's electrode fingers connected to a signal line and which isdisposed at an outermost location, to the center of an innermostelectrode finger of the reflector 11 d, and the distance Q is defined asthe distance from the center of an electrode finger, which is one of theIDT 11 a's electrode fingers connected to the signal line and which isdisposed at the outermost location on the opposite side, to the centerof an innermost electrode finger of the reflector 11 e.

FIG. 14 is a plan view schematically illustrating the electrodestructure of a third preferred embodiment of a surface acoustic wavefilter device. Also in this third preferred embodiment, three surfaceacoustic wave filter elements 31 to 33 are disposed on a piezoelectricsubstrate. The respective surface acoustic wave filter elements 31 to 33are preferably formed in the same manner as in the second preferredembodiment. Similar elements to those in the second preferred embodimentare denoted by similar reference numerals and they are not describedfurther herein.

In this third preferred embodiment, the manner of connecting the surfaceacoustic wave filter element 31 to the surface acoustic wave filterelements 32 and 33 is different from that in the second preferredembodiment.

That is, in the third preferred embodiment, the IDTs disposed at outerlocations in the first to third surface acoustic wave filter elements 31to 33, that is, the IDTs 31 b, 31 c, 32 b, 32 c, 33 b, and 33 c, are notgrounded but floated.

More specifically, one comb-shaped electrode of the second IDT 31 b ofthe first surface acoustic wave filter element 31 is connected to oneend of each of the second and third IDTs 32 b and 32 c of the secondsurface acoustic wave filter element 32. Furthermore, the other end ofthe IDT 31 b is connected to the other end of each of the IDTs 32 b and32 c of the second surface acoustic wave filter element 32. Similarly,one end of the third IDT 31 c of the first surface acoustic wave filterelement 31 is connected to one end of each of the second and third IDTs33 b and 33 c of the third surface acoustic wave filter element 33, andthe second end of the IDT 31 c is connected to the second end of each ofthe IDTS 33 b and 33 c.

Reflectors are denoted by reference numerals 31 d, 31 e, 32 d, 32 e, 33d, and 33 e. Except for the above, the surface acoustic wave filterdevice of the present preferred embodiment is similar to that of thesecond preferred embodiment.

Thus, the surface acoustic wave filter device of the third preferredembodiment operates in a manner similar to the surface acoustic wavefilter device of the second preferred embodiment, and therefore similaradvantages are obtained. The above-described connection structure usedherein allows a great reduction in the number of bonding pads forconnection to ground and also allows a reduction in the size of thesurface acoustic wave filter device. Furthermore, the above-describedconnection structure allows a reduction in parasitic capacitanceassociated with the bonding pads and the interconnections to the bondingpads.

FIG. 15 is a plan view schematically illustrating the electrodestructure of a fourth preferred embodiment of a surface acoustic wavefilter device according to the present invention.

In the surface acoustic wave filter device of the present preferredembodiment, first and second surface acoustic wave filter elements 41and 42 are disposed on a piezoelectric substrate which is not shown inthe figure. A substrate made of piezoelectric ceramics or piezoelectricsingle crystal may be used as the piezoelectric substrate. In thispreferred embodiment, a 36° Y-X LiTaO₃ substrate is preferably used.

The first and second surface acoustic wave filter elements 41 and 42 areboth resonator-type surface acoustic wave filter elements each havingthree IDTs 41 a to 41 c or 42 a to 42 c.

A first end of the first IDT 41 a disposed at the central location inthe surface acoustic wave filter element 41 and a first end of the firstIDT 42 a disposed at the central location in the second surface acousticwave filter element 42 are connected in common to an input terminal 4.

The second end of each of the first IDTs 41 a and 42 a is grounded. Onthe other hand, outer IDTs, that is, IDTs 41 b and 41 c are connected toan output terminal 5, and one end of each of the outer IDTs, that is,the second and third IDTs 42 b and 42 c, is connected to an outputterminal 6. The other end of each of the second and third IDTs 41 b, 41c, 42 b, and 42 c is grounded.

Reflectors 41 d and 41 e are respectively disposed on opposite sides ofthe area where the IDTs 41 a to 41 c are disposed, and reflectors 42 dand 42 e are respectively disposed on opposite sides of the area wherethe IDTs 42 a to 42 c are disposed.

In the present preferred embodiment, the transmission phasecharacteristic of the first surface acoustic wave filter element 41 isdifferent by about 180° from that of the second surface acoustic wavefilter element 41.

More specifically, in the first surface acoustic wave filter element 41,the interdigital overlapping length W of the electrode finger ispreferably equal to about 31λ, the IDT 41 a has 16 pairs of electrodefingers, and the IDTs 41 b and 41 c each have 11 pairs of electrodefingers. λI of the IDTs 41 a to 41 c is preferably equal to about 4.2μm. The reflectors 41 d and 41 e each have 120 electrode fingers, andthe wavelength λR of the reflectors 41 d and 42 e is preferably equal toabout 4.3 μm. The first space GI₁ between the IDT 41 a and the IDT 41 bor 41 c is preferably equal to about 1.75λR.

The second surface acoustic wave filter element 42 is preferably formedin a manner similar to the first surface acoustic wave filter element 41except that the second space GI₂ between the IDT 42 a and the IDT 42 bor 42 c is equal to about 2.25λR. The first and second spaces arepreferably different from each other as described above so that thefirst surface acoustic wave filter element 41 and the second surfaceacoustic wave filter element 42 have substantially the same transmissionamplitude characteristic but have a transmission phase characteristicdifferent by about 180°.

The operation of the surface wave acoustic filter element of the presentpreferred embodiment is described below for the case where the inputterminal 4 is used as an unbalanced input terminal and the outputterminals 5 and 6 are used as balanced output terminals.

When an electrical signal is input to the input terminal 4, signalshaving the same phase and amplitude are applied to the first and secondsurface acoustic wave filter elements 41 and 42. These signals areapplied to the IDTs 41 a and 42 a and thus surface acoustic waves areexcited. The surface acoustic waves propagate in a direction that issubstantially perpendicular to the direction in which the electrodefingers extend and are reflected by the reflectors 41 d and 41 e or thereflectors 42 d and 42 e. The reflected surface acoustic waves interferewith the excited surface acoustic waves, and thus standing waves arecreated between the two reflectors 41 d and 41 e and between the tworeflectors 42 d and 42 e. As a result, resonance with very high Qoccurs. The excited standing waves are received by the IDTs 41 b, 41 c,42 b, and 42 c connected to the output terminal 5 or 6 and converted toelectrical signals. Herein, the output signals are determined by therelative positional relationship between the excited standing waves andthe IDTs 41 b, 41 c, 42 b, and 42 c disposed on the output side.

In the present preferred embodiment, the first space between the IDT 41a and the IDT 41 b or 41 c in the surface acoustic wave filter element41 and the second space between the IDT 42 a and the IDT 42 b or 42 c inthe second surface acoustic wave filter element 42 are preferablydifferent from each other by about 0.50 times the wavelength of thesurface acoustic wave. As a result, the signal output from the firstsurface acoustic wave filter element 41 and the signal output from thesecond surface acoustic wave filter element 42 become opposite in phase.

That is, the surface acoustic wave filter elements 41 and 42 havetransmission phase characteristic different by 180° from each other, andelectrical signals which are substantially equal in amplitude butdifferent in phase by 180° are output from the surface acoustic wavefilter elements 41 and 42 to the respective output terminals 5 and 6serving as balanced output terminals.

In the present preferred embodiment, a one-stage filter is provided toinclude the two surface acoustic wave filter elements 41 and 42, whereinthe one-stage structure makes it possible to reduce the insertion losswithin the band to a very low level.

The filter characteristic of the surface acoustic wave filter device ofthe fourth preferred embodiment is shown in FIG. 16. FIG. 16 indicatesthat a reduction in loss within the passband is achieved.

The results shown in FIGS. 5 and 6 also apply to this fourth preferredembodiment, and thus the difference between the first space and thesecond space is preferably within the range from about 0.48λ to about0.525λ.

A wideband characteristic can be achieved without having degradation inthe degree of balance, by setting the first and second spaces within acombination of the range from about (n/2+1.22)×λ to (n/2+1.33)×λ (n isan integer of 0 to 4) and the range from about (n/2+1.72)×λ to(n/2+1.83)×λ (n is an integer of 0 to 4).

Also in this fourth preferred embodiment, as in the first preferredembodiment, if one or more electrode fingers are inserted in the firstand second spaces thereby increasing the electrode covering ratio,propagation of leaky waves becomes dominant, and excitation andpropagation of SSBW are suppressed. That is, it is possible to provide alow-loss surface acoustic wave filter device by preferably setting theelectrode covering ratio in the first and second spaces to be equal toor greater than about 50% and more preferably equal to or greater thanabout 63%.

In the present preferred embodiment, the locations of the IDTs 42 b and42 c disposed on the output side in the second surface acoustic wavefilter element 42 are shifted by about 0.5 times the wavelength of thesurface acoustic wave with respect to the locations of the IDTs 41 b and41 c disposed on the output side in the first surface acoustic wavefilter element so as to make the phases opposite to each other asdescribed above.

Herein, if the distance between the two reflectors 41 d and 41 e in thefirst surface acoustic wave filter element 41 and the distance betweenthe reflectors 42 d and 42 e in the second surface acoustic wave filterelement 42 are different from each other, the amplitude distribution ofthe standing wave becomes different between the first and second surfaceacoustic wave filter elements. Such a difference can result invariations in the resonance characteristic and the filtercharacteristic. To avoid the above problem, it is desirable that thedistance P₁ between the reflectors 41 d and 41 e and the distance Q₁between the reflectors 42 d and 42 e be substantially equal to eachother thereby suppressing the degradation in the degree of balance.

Although grating-type reflectors are preferably used as the reflectors41 d to 42 e also in this fourth preferred embodiment, other types ofreflectors may also be used. For example, reflectors using reflection atan end surface of a chip may be used.

Furthermore, because the IDT 41 a disposed at the central location inthe surface acoustic wave filter element 41 and the IDT 42 a disposed atthe central location in the second surface acoustic wave filter element42 are connected in common via an electrode pattern on the piezoelectricsubstrate and further connected to the unbalanced input terminal 4, theparasitic capacitance associated with the surface acoustic wave filterelement 41 and the parasitic capacitance associated with the surfaceacoustic wave filter element 42 are shared by each other. This resultsin a further improvement in the degree of balance.

FIG. 17 is a plan view schematically illustrating the electrodestructure of a fifth preferred embodiment of a surface acoustic wavefilter device.

Also in the present preferred embodiment, as in the fourth preferredembodiment, two resonator-type surface acoustic wave filter elements areused. That is, first and second surface acoustic wave filter elements 51and 52 are disposed on a piezoelectric substrate. In the first andsecond surface acoustic wave filter elements 51 and 52, floatingelectrode fingers 53 a to 53 d are respectively disposed in the spacesbetween the central IDT 51 a or 52 a and the outer second and third IDTs51 b and 51 c or 52 b and 52 c. The IDTs 51 a to 51 c and the reflectors51 d and 51 e are constructed in a manner similar to the IDTs 41 a to 41c and the reflectors 45 d and 45 e in the surface acoustic wave filterelement 41 of the fourth preferred embodiment. The IDTs 52 a to 52 c andthe reflectors 52 d and 52 e in the second surface acoustic wave filterelement 51 are constructed in a manner similar to the IDTs 42 a to 42 cand the reflectors 42 d and 42 e in the second surface acoustic wavefilter element 42 of the fourth preferred embodiment.

In the present preferred embodiment, as described above, floatingelectrode fingers 53 a to 53 d are formed separately from the IDTs so asto achieve an electrode covering ratio greater than about 50% for thespaces between IDTs.

FIG. 18 is a plan view schematically illustrating a surface acousticwave filter device according to a sixth preferred embodiment. A surfaceacoustic wave filter element 61 is disposed on a piezoelectric substratethat is not shown in the figure. In the present preferred embodiment, a36° Y-X LiTaO₃ substrate is preferably used as the piezoelectricsubstrate. Note that LiTaO₃ substrates cut in different orientations orpiezoelectric substrates made of other types of piezoelectric materialsmay also be used.

The surface acoustic wave filter element 61 includes three IDTs 61 a to61 c disposed in the same direction as the direction in which thesurface acoustic wave propagates. Reflectors 61 d and 61 e are disposedat respective opposite ends in the area where the IDTs 61 a to 61 c aredisposed.

In the present preferred embodiment, one end of the first IDT 61 a atthe central location is connected to an input terminal 4 in the form ofan unbalanced input terminal. The other end of the IDT 61 a is grounded.One end of each of the second and third IDTs 61 b and 61 c at outerlocations is connected to an output terminal 5 or 6 in the form ofunbalanced terminals, and the other end of each of the second and thirdIDTs 61 b and 61 c is grounded. Reflectors 61 d and 61 e are constructedto define grating reflectors. Note that other types of reflectors mayalso be used.

The interdigital overlapping length W of the electrode finger of eachIDT 61 a to 61 c is preferably equal to about 31λ. The IDT 61 a has 16pairs of electrode fingers, and IDTs 61 b and 61 c each have 11 pairs ofelectrode fingers. In the IDTs 61 a to 61 c, the wavelength λI of thesurface acoustic wave is preferably equal to about 4.2 μm.

The reflectors 61 d and 61 e each have 120 electrode fingers, and thewavelength λR is preferably equal to about 4.3 μm.

The first space JI₁ between the IDT 61 a and the IDT 61 b is preferablyequal to about 1.75λR, and the second space JI₂ between the IDT 61 a andthe IDT 61 c is preferably equal to about 2.25λR,

In the surface acoustic wave filter device of the present preferredembodiment, when an electrical signal is applied to the IDT 61 a via theinput terminal 4, a standing wave is created between the reflectors 61 dand 61 e as in the first to fifth preferred embodiments. The standingwave allows resonance to occur with very high Q. The excited standingwave is received by the IDTs 61 b and 61 c and output via the outputterminals 5 and 6.

Also in the present preferred embodiment, the output signals aredetermined by the relative positional relationship between the excitedstanding wave and the IDTs 61 b and 61 c disposed on the output side. Inthe present preferred embodiment, the first space between the IDT 61 aand the IDT 61 b and the second space between the IDT 61 a and the IDT61 c is different from each other by about 0.50 times the wavelength ofthe surface acoustic wave so that the output signals of the IDTs 61 band 61 c become opposite in phase to each other.

That is, the electrical signal output from the IDT 61 b and theelectrical signal output from the IDT 61 c are different in transmissionphase characteristic by about 180°, and thus electrical signals whichare substantially equal in amplitude but different in phase by about180° are output from the output terminals 5 and 6.

The results shown in FIGS. 5 and 6 also apply to the present preferredembodiment, and thus the difference between the first space and thesecond space is preferably within the range from about 0.48λ to about0.525λ. A wideband characteristic can be achieved without havingdegradation in the degree of balance, by setting the respective spacesJI₁ and JI₂ within a combination of the range from about (n+1.22)×λ to(n+1.33)×λ (n is an integer of 0 to 4) and the range from about(n+0.72)×λ to (n+0.83)×λ (n is an integer of 0 to 4).

Furthermore, in the present preferred embodiment, the innermostelectrode finger of each of the IDTs 61 b and 61 c is arranged to have agreater width so that the electrode covering ratios in the spaces JI₁and JI₂ between IDTs become equal to about 0.63 thereby reducing thepropagation loss in the spaces JI₁ and JI₂ between IDTs. This avoidsdegradation in the amplitude balance due to the difference between thefirst and second spaces.

Furthermore, the distances P and Q from the central IDT 61 a to therespective reflectors 61 d and 61 e are preferably substantially equalto each other so that the excited amplitude distribution of the standingwave does not become asymmetrical and so that degradation in the degreeof balance is prevented.

FIG. 19 is a plan view schematically illustrating a surface acousticwave filter device according to a seventh preferred embodiment of thepresent invention. In this seventh preferred embodiment, floatingelectrode fingers 72 and 73 are respectively disposed in the spacesbetween a first IDT 71 a at the central location and second and thirdIDTs 71 b and 71 c at outer locations. Except for the above, the surfaceacoustic wave filter device of the present preferred embodiment is thesame as that of the sixth preferred embodiment. Because the IDT 71 a to71 c and the reflectors 71 d and 71 e are arranged in the same manner asthose in the surface acoustic wave filter device of the sixth preferredembodiment, the surface acoustic wave filter device of the presentpreferred embodiment has similar advantages to those obtained in thesixth preferred embodiment.

Furthermore, the floating electrode fingers 72 and 73 result in anincrease in the electrode coverage ratio in the first and second spaces,which in turn results in a reduction in the propagation loss.

FIG. 20 is a plan view schematically illustrating a surface acousticwave filter device according to an eighth preferred embodiment of thepresent invention. First and second surface acoustic wave filterelements 81 and 82 are disposed on a piezoelectric substrate that is notshown in the figure. The first and second surface acoustic wave filterelements 81 and 82 are preferably formed in the same manner as in thesurface acoustic wave filter device of the fourth preferred embodimentexcept that a first surface acoustic wave resonator 83 is connectedbetween an input terminal 4 and first IDTs 81 a and 82 a at centrallocations in respective first and second surface acoustic wave filterdevices 81 and 82 and except that one pair of surface acoustic waveresonators 84 and 85 are connected from the second and third IDTs 81 band 81 c or 82 b and 82 c at outer locations in the respective first andsecond surface acoustic wave filter elements 81 and 82 to the respectiveoutput terminals 5 and 6. Reflectors are denoted by reference numerals81 d, 81 e, 82 d, and 82 e.

The above-described first surface acoustic wave resonator 83 includesone IDT 83 a and reflectors of the grating type (not shown) disposed onrespective opposite sides of the IDT 83 a.

In the first surface acoustic wave resonator 83, the interdigitaloverlapping length W of the electrode finger of the IDT 83 a ispreferably equal to about 20λ, and the number N of pairs of electrodefingers is preferably equal to 80. The wavelength λI of the IDT ispreferably equal to about 4.20 μm. The reflectors which are not shown inthe figure have 120 electrode fingers.

The second and third surface acoustic wave resonators 84 and 85connected to the output terminals 5 and 6 are preferably formed in amanner similar to the first surface acoustic wave resonator 83.

In the present preferred embodiment, the first and third surfaceacoustic wave resonators 83 to 85 connected in the above-describedmanner result in a great increase in attenuation outside the passbandcompared with the fourth preferred embodiment, as shown in FIG. 21. InFIG. 21, the solid line represents the filter characteristic of thesurface acoustic wave filter device of the eighth preferred embodiment,and the broken line represents the filter characteristic of the surfaceacoustic wave filter device of the fourth preferred embodiment.

FIG. 22 is a plan view schematically illustrating the electrodestructure of a surface acoustic wave filter device according to a ninthpreferred embodiment. The surface acoustic wave filter device of thisninth preferred embodiment has a structure which is obtained byinserting first to third surface acoustic wave resonators 93 to 95 atthe input and output sides of the surface acoustic wave filter device ofthe sixth preferred embodiment in a manner similar to the eighthpreferred embodiment.

The surface acoustic wave filter element 91 is preferably formed in thesame manner as the surface acoustic wave filter element 61 of the sixthpreferred embodiment. The first surface acoustic wave resonator 93connected between the input terminal 4 and the first IDT 91 a at thecentral location and the second and third surface acoustic waveresonators 94 and 95 connected between the output terminals 5 and 6 andthe second and third IDTs 91 b and 91 c are preferably formed in thesame manner as the surface acoustic wave resonator 83 to 85 of theeighth preferred embodiment.

Also in the present preferred embodiment, as in the eighth preferredembodiment, the first and third surface acoustic wave resonatorsconnected between the input terminal and the input end of the surfaceacoustic wave filter element or between the output terminal and theoutput end of the surface acoustic wave filter element result in anincrease in attenuation near the edges of the passband, in particular,near the upper edge.

FIG. 23 is a plan view schematically illustrating the electrodestructure of a surface acoustic wave filter device according to a tenthpreferred embodiment. The surface acoustic wave filter device of thepresent preferred embodiment has a structure obtained by adding a fourthsurface acoustic wave resonator 101 to the surface acoustic wave filterdevice of the eighth preferred embodiment such that the fourth surfaceacoustic wave resonator 101 is inserted between the second surfaceacoustic wave resonator 84 and the output terminal 6. In other words,the fourth surface acoustic wave resonator 101 is connected in parallelto the output terminals 5 and 6. The fourth surface acoustic waveresonator 101 includes one IDT and two grating-type reflectors disposedon the respective opposite sides of the IDT, although the grating-typereflectors are not shown in the figure. The fourth surface acoustic waveresonator 101 is arranged such that the interdigital overlapping lengthW of the electrode finger of the IDT is preferably equal to about 15λ,the IDT has 50 pairs of electrode fingers, the wavelength λI of the IDTis preferably equal to about 4.40 μm, and the reflectors have 120electrode fingers.

By adding the fourth surface acoustic wave resonator 101 to the surfaceacoustic wave filter device of the eighth preferred embodiment in themanner according to the present preferred embodiment, a ladder-typefilter circuit is disposed on the side of the balanced output terminals5 and 6. If this ladder-type filter circuit is arranged such that itsattenuation poles are located near the lower and upper edges of thepassband of the surface acoustic wave filter element 81, greaterattenuation and better selectivity can be achieved.

Because the surface acoustic wave resonator 101 is connected in the formof a bridge between the balanced output terminals 5 and 6, theinfluences imposed upon the balanced terminals 5 and 6 are substantiallyequal, and thus factors which would otherwise cause degradation in thedegree of balance are cancelled out. Therefore, the attenuation outsidethe passband can be increased without causing degradation in the degreeof balance.

In the first to tenth preferred embodiments described above, surfaceacoustic wave filter devices have been described only in terms of theelectrode structure disposed on the piezoelectric substrate. However,the surface acoustic wave filter device according to various preferredembodiments of the present invention can be constructed in the form of achip-type surface acoustic wave filter device by using various packagestructures.

In an eleventh preferred embodiment described below, a surface acousticwave filter device is provided in the form of a component housed in apackage. A surface acoustic wave filter element is obtained by disposedelectrodes on a piezoelectric substrate 102 according to preferredembodiments of the present invention, as shown in FIG. 24A. The surfaceacoustic wave filter element is housed in a package 103 having a cavity103 a.

In the present preferred embodiment, the piezoelectric substrate 102used to produce the surface acoustic wave filter element preferably hasa substantially rectangular plate shape, and a plurality of IDTs andplurality of electrode pads 104 a for input, output and ground arearranged symmetrically with respect to an axis X passing through thecenter of the piezoelectric substrate 102.

The package 103 also preferably has a substantially rectangular shape inplan view and has a symmetry axis Y passing through the center. Aplurality of electrodes pads 104 b are provided on an inner bottomsurface of the cavity 103 a and arranged symmetrically with respect toan axis Y. The electrode pads 104 a on the piezoelectric substrate 102and the electrode pads 104 b on the inner bottom of the cavity 103 a ofthe package 103 are bonded by flip chip bonding so that the surfaceacoustic wave filter provided on the piezoelectric substrate 102 isbonded on the bottom of the cavity 103 in a face-down manner and fixedto the package with the axis X and axis Y coincide.

Alternatively, the piezoelectric substrate 102 may be firmly placed inthe package 103 face-up such that the symmetry axis X of thepiezoelectric substrate 102 and the symmetry axis Y of the package 103become coincident with each other. In the case, although not shown inFIG. 24A, the surface acoustic wave filter element is connected toelectrode pads provided on the package 103 via bonding wires. Theelectrode pads and the bonding wires are also disposed in an axiallysymmetric fashion with respect to the symmetry axis X or Y.

As a result of the coincidence between the symmetry axis X of thepiezoelectric substrate 102 and the symmetry axis Y of the package 103,interconnection lines which are disposed on the surface acoustic wavefilter and which are connected to the respective balanced outputterminals become substantially equal in terms of the electrical lengthand parasitic capacitance. Therefore, degradation in the degree ofbalance is suppressed.

Furthermore, because the package has the axially symmetric structurewith respect to the symmetry axis Y passing through the center of thepackage, the electrical length and parasitic capacitance ofinterconnections which are disposed in the package and which areconnected to the respective balanced terminals become equal for eachbalanced terminal. This also results in suppression of degradation inthe degree of balance. Thus, factors which can cause degradation in thedegree of balance can be reduced to very low levels, and a surfaceacoustic wave filter device which has a balance-unbalance conversioncapability and which is excellent in terms of the degree of balance isobtained.

In the case where the electrical connections are made via flip-chipbonding instead of wire bonding, since no bonding wire is required, isit possible to symmetrically arrange the electric length and straycapacitance with respect to the electric connection between thepiezoelectric substrate and package, thereby increasing the balance ofthe surface acoustic wave filter.

In the case where the wire bonding is used, the same effects can beobtained by disposing the electrode pads and the bonding wires atlocations axially symmetric with respect to the symmetry axis X or Y asdescribed above.

In the present embodiment, it is also advantageous that externalterminals such as an external input terminal, external output terminalsare arranged symmetrically on an outer bottom surface of a package.

As shown in FIG. 24B, a package 105 in which a surface acoustic wavefilter according to a preferred embodiment of the present invention isplaced has an external input terminal 106, external output terminals 107a and 107 b and external grounded terminals 108 a to 108 c on the outerbottom surface of the package 105. The package 105 has a substantiallyrectangular bottom surface, and the external input terminal 106 islocated on a symmetric axis Y of the package 105. The external outputterminals 107 a and 107 b are disposed on the outer bottom surface ofthe package 105 so as to be located symmetrically with respect to theexternal input terminal 106 or the axis Y. The external groundedterminals 108 a to 108 c are also arranged symmetrically with respect tothe symmetric axis Y. More specifically, the external grounded terminals108 a and 108 b are located such that the external input terminal 106 islocated at the middle point between the external grounded terminals 108a and 108 b, and the external grounded terminal 108 c is positioned onthe symmetric axis Y.

These external terminals are electrically connected to theabove-explained electrode pads provided on the inner bottom of thepackage, and the electrode pads are also connected to the terminals ofthe surface acoustic wave filter placed in the package 105 by flip-chipbonding or wire bonding.

For example, in the case where the surface acoustic wave filter isconstructed according to the first preferred embodiment of the presentinvention shown in FIG. 1, the external input terminal 106 iselectrically connected to the input terminal 4, and the external outputterminals 107 a and 107 b are electrically connected to the outputterminals 6 and 5, respectively. The external grounded terminal 108 c ispreferably connected to the IDTs 2 a and 3 a, and the external groundedterminals 108 a and 108 b are connected to the other IDTs to begrounded.

As shown in FIG. 24C, the external grounded terminals 108 a and 108 bmay be placed between the external input terminal 106 and the externaloutput terminals 107 a and 107 b, respectively.

According to the structure, since the two external output terminals arearranged symmetrically with respect to the external input terminal, thedegree of balance is further improved. When the external groundedterminal is provided between the external output terminals, the degreeof balance is still further improved. On the other hand, when the twoexternal grounded terminals are provided between the external inputterminal and the two external output terminals, it is possible to reducethe direct propagation signal component between the input terminal andthe output terminals.

FIG. 25 is a plan view schematically illustrating a twelfth preferredembodiment of a surface acoustic wave filter including unbalance-balancesurface acoustic wave filters which are arranged on the samepiezoelectric substrate so as to have different frequencycharacteristics. In this surface acoustic wave. filter 111, surfaceacoustic wave filter devices 113 and 114 having the same structure asthe surface acoustic wave filter device shown in FIG. 20 are disposed onthe same piezoelectric substrate 112. For example, the surface acousticwave filter device 113 may be a 900-MHz bandpass filter and the surfaceacoustic wave filter device 114 may be a 1900-MHz bandpass filter. Theformation of two unbalance-balance surface acoustic wave filter devices113 and 114 on the same piezoelectric substrate 112 results in areduction in the size of the bandpass filter.

In FIG. 25, electrode pads disposed on the piezoelectric substrate areconnected to an electrode pattern or a ground electrode pattern disposedon a package via bonding wires. However, instead of the wire bonding,another technique may also be used to achieve the electricalconnections.

FIG. 26 is a schematic diagram illustrating an antenna duplexer usingthe surface acoustic wave filter device 111 shown in FIG. 25. In thisantenna duplexer, the input ends of the surface acoustic wave filterdevices 113 and 114 are connected in common to an antenna ANT. Theoutput ends of the surface acoustic wave filter devices 113 and 114functions as a transmission output end Tx and a reception output end Rx,respectively.

In FIG. 25, filters having different frequency characteristics aredisposed on the same piezoelectric substrate 112. Alternatively, surfaceacoustic wave filter devices 113 and 114 having different frequencycharacteristics may be disposed on different piezoelectric substrates112 a and 112 b, as shown in FIG. 27. In this case, the surface acousticwave filter devices 113 and 114 disposed on the different piezoelectricsubstrates 112 a and 112 b are housed in a package 116.

While preferred embodiments of the invention have been disclosed,various modes of carrying out the principles disclosed herein arecontemplated as being within the scope of the following claims.Therefore, it is understood that the scope of the present invention isnot to be limited except as otherwise set forth in the claims.

What is claimed is:
 1. A surface acoustic wave filter device comprising:a piezoelectric substrate; and first and second surface acoustic wavefilter elements disposed on said piezoelectric substrate; wherein saidfirst surface acoustic wave filter element includes a plurality of IDTsdisposed along the surface acoustic wave propagation direction; saidsecond surface acoustic wave filter element includes a plurality of IDTsdisposed along the surface acoustic wave propagation direction; saidsecond surface acoustic wave filter element is arranged such that thetransmission amplitude characteristic of said second surface acousticwave filter element is substantially equal to that of said first surfaceacoustic wave filter element and such that the transmission phasecharacteristic of said second surface acoustic wave filter element isdifferent by about 180° from that of said first surface acoustic wavefilter element; one end of each of the first and second surface acousticwave filter elements define unbalanced terminals and the other end ofeach of the first and second surface acoustic wave filter elementsdefine balanced terminals; each of the first and second surface acousticwave filter element has three IDTs and a second space defined in thesecond surface acoustic wave filter element between an IDT connected tothe unbalanced terminal and an IDT connected to a balanced terminal isdifferent by about 0.48λ to about 0.525λ from a first space defined inthe first surface acoustic wave filter element between an IDT disposedat a central location and an IDT disposed on either side, where λ is thewavelength of the surface acoustic wave; and said first space is equalto about(n/2+1.22)×λ to about (n/2+1.33)×λ (n is an integer of 0 to 4),and said second space is equal to about (n/2+1.72)×λ to about(n/2+1.83)×λ (n is an integer of 0 to 4).
 2. A surface acoustic wavefilter device according to claim 1, wherein said piezoelectric substrateis an approximately 36° to 44°-rotated Y-cut LiTaO₃ substrate made of aLiTaO₃ single crystal with an orientation rotated about the X axis fromthe Y axis to the Z axis within the range from about 36° to about 44°,and at least one electrode finger is inserted in at least one of thefirst and second spaces in the first and second surface acoustic wavefilter elements so that the electrode covering ratio of the space inwhich said electrode finger is inserted becomes equal to or greater thanabout 50%.
 3. A surface acoustic wave filter device according to claim2, wherein said electrode covering ratio is equal to or greater thanabout 63%.
 4. A surface acoustic wave filter device according to claim1, wherein first and second reflectors are disposed on respectiveopposite sides in the surface acoustic wave propagation direction of thearea in which said plurality of IDTs of the first surface acoustic wavefilter element are disposed, and third and fourth reflectors disposed onrespective opposite sides, in the surface acoustic wave propagationdirection, of the area in which said plurality of IDTs of the secondsurface acoustic wave filter element are disposed, wherein the distancebetween the first reflector and the second reflector is substantiallyequal to the distance between the third reflector and the fourthreflector.
 5. A surface acoustic wave filter device according to claim1, wherein the unbalanced-side terminal of the first surface acousticwave filter element and the unbalanced-side terminal of the secondsurface acoustic wave filter element are connected to each other via anelectrode pattern on the piezoelectric substrate.
 6. A surface acousticwave filter device according to claim 1, further comprising a seriesresonator connected on the unbalanced terminal side.
 7. A surfaceacoustic wave filter device according to claim 1, further comprisingsurface acoustic wave resonators connected in series to the respectiveterminals on the balanced terminal side.
 8. A surface acoustic wavefilter device according to claim 1, further comprising a ladder-typesurface acoustic wave filter connected in a cascade fashion on thebalanced terminal side.
 9. A surface acoustic wave filter deviceaccording to claim 1, further comprising a package which has a cavityfor housing a chip including the piezoelectric substrate on which thesurface acoustic wave filter elements are disposed, said package havingon an inner bottom of the cavity electrodes pads to be electricallyconnected to electrode patterns on the chip, wherein at least one of theelectrode pattern disposed on the piezoelectric substrate, the package,or the electrode pads has a substantially axially symmetric structure.10. A surface acoustic wave filter device according to claim 9, whereinat least two of the electrode pattern, the package, and the electrodepads have structures which are substantially axially symmetric withrespect to the same symmetry axis.
 11. A surface acoustic wave filterdevice according to claim 1, further comprising a package which has acavity for housing a chip including the piezoelectric substrate on whichthe surface acoustic wave filter elements are disposed, said packagehaving on an outer bottom of the cavity an external input terminal andtwo external output terminals, such that the two external outputterminals are symmetric with respect to said external input terminal.12. A surface acoustic wave filter device according to claim 11, furthercomprising a external grounded terminal provided between said twoexternal output terminals.
 13. A surface acoustic wave filter deviceaccording to claim 11, further comprising two external groundedterminals provided between said external input terminal and said twoexternal output terminals, respectively.
 14. A duplexer comprising asurface acoustic wave filter according to claim
 1. 15. A communicationdevice comprising a duplexer according to claim
 14. 16. A surfaceacoustic wave filter device comprising: a piezoelectric substrate; andfirst and second surface acoustic wave filter elements disposed on saidpiezoelectric substrate; wherein said first surface acoustic wave filterelement includes a plurality of IDTs disposed along the surface acousticwave propagation direction; said second surface acoustic wave filterelement includes a plurality of IDTs disposed along the surface acousticwave propagation direction; said second surface acoustic wave filterelement is arranged such that the transmission amplitude characteristicof said second surface acoustic wave filter element is substantiallyequal to that of said first surface acoustic wave filter element andsuch that the transmission phase characteristic of said second surfaceacoustic wave filter element is different by about 180° from that ofsaid first surface acoustic wave filter element; one end of each of thefirst and second surface acoustic wave filter elements define unbalancedterminals and the other end of each of the first and second surfaceacoustic wave filter elements define balanced terminals; each of thefirst and second surface acoustic wave filter element has three IDTs anda second space defined in the second surface acoustic wave filterelement between an IDT connected to the unbalanced terminal and an IDTconnected to a balanced terminal is different by about 0.48λ to about0.525λ from a first space defined in the first surface acoustic wavefilter element between an IDT disposed at a central location and an IDTdisposed on either side, where λ is the wavelength of the surfaceacoustic wave; and said first space is equal to about (n/2+1.22)×λ toabout (n/2+1.33)×λ (n is an integer of 0 to 2), and said second space isequal to about (n/2+1.72)×λ to about (n/2+1.83)×λ (n is an integer of 0to 2).
 17. A surface acoustic wave filter device according to claim 16,wherein said piezoelectric substrate is an approximately 36° to44°-rotated Y-cut LiTaO₃ substrate made of a LiTaO₃ single crystal withan orientation rotated about the X axis from the Y axis to the Z axiswithin the range from about 36° to about 44°, and at least one electrodefinger is inserted in at least one of the first and second spaces in thefirst and second surface acoustic wave filter elements so that theelectrode covering ratio of the space in which said electrode finger isinserted becomes equal to or greater than about 50%.
 18. A surfaceacoustic wave filter device according to claim 17, wherein saidelectrode covering ratio is equal to or greater than about 63%.
 19. Asurface acoustic wave filter device according to claim 16, wherein firstand second reflectors are disposed on respective opposite sides in thesurface acoustic wave propagation direction of the area in which saidplurality of IDTs of the first surface acoustic wave filter element aredisposed, and third and fourth reflectors disposed on respectiveopposite sides, in the surface acoustic wave propagation direction, ofthe area in which said plurality of IDTs of the second surface acousticwave filter element are disposed, wherein the distance between the firstreflector and the second reflector is substantially equal to thedistance between the third reflector and the fourth reflector.
 20. Asurface acoustic wave filter device according to claim 16, wherein theunbalanced-side terminal of the first surface acoustic wave filterelement and the unbalanced-side terminal of the second surface acousticwave filter element are connected to each other via an electrode patternon the piezoelectric substrate.
 21. A surface acoustic wave filterdevice according to claim 16, further comprising a series resonatorconnected on the unbalanced terminal side.
 22. A surface acoustic wavefilter device according to claim 16, further comprising surface acousticwave resonators connected in series to the respective terminals on thebalanced terminal side.
 23. A surface acoustic wave filter deviceaccording to claim 16, further comprising a ladder-type surface acousticwave filter connected in a cascade fashion on the balanced terminalside.
 24. A surface acoustic wave filter device according to claim 16,further comprising a package which has a cavity for housing a chipincluding the piezoelectric substrate on which the surface acoustic wavefilter elements are disposed, said package having on an inner bottom ofthe cavity electrodes pads to be electrically connected to electrodepatterns on the chip, wherein at least one of the electrode patterndisposed on the piezoelectric substrate, the package, or the electrodepads has a substantially axially symmetric structure.
 25. A surfaceacoustic wave filter device according to claim 24, wherein at least twoof the electrode pattern, the package, and the electrode pads havestructures which are substantially axially symmetric with respect to thesame symmetry axis.
 26. A surface acoustic wave filter device accordingto claim 16, further comprising a package which has a cavity for housinga chip including the piezoelectric substrate on which the surfaceacoustic wave filter elements are disposed, said package having on anouter bottom of the cavity an external input terminal and two externaloutput terminals, such that the two external output terminals aresymmetric with respect to said external input terminal.
 27. A surfaceacoustic wave filter device according to claim 26, further comprising aexternal grounded terminal provided between said two external outputterminals.
 28. A surface acoustic wave filter device according to claim26, further comprising two external grounded terminals provided betweensaid external input terminal and said two external output terminals,respectively.
 29. A duplexer comprising a surface acoustic wave filteraccording to claim
 16. 30. A communication device comprising a duplexeraccording to claim
 29. 31. A surface acoustic wave filter devicecomprising: a piezoelectric substrate; and first and second surfaceacoustic wave filter elements disposed on said piezoelectric substrate;wherein said first surface acoustic wave filter element includes aplurality of IDTs disposed along the surface acoustic wave propagationdirection; said second surface acoustic wave filter element includes aplurality of IDTs disposed along the surface acoustic wave propagationdirection; said second surface acoustic wave filter element is arrangedsuch that the transmission amplitude characteristic of said secondsurface acoustic wave filter element is substantially equal to that ofsaid first surface acoustic wave filter element and such that thetransmission phase characteristic of said second surface acoustic wavefilter element is different by about 180° from that of said firstsurface acoustic wave filter element; one end of each of the first andsecond surface acoustic wave filter elements define unbalanced terminalsand the other end of each of the first and second surface acoustic wavefilter elements define balanced terminals; each of the first and secondsurface acoustic wave filter element has three IDTs and a second spacedefined in the second surface acoustic wave filter element between anIDT connected to the unbalanced terminal and an IDT connected to abalanced terminal is different by about 0.48λ to about 0.525λ from afirst space defined in the first surface acoustic wave filter elementbetween an IDT disposed at a central location and an IDT disposed oneither side, where λ is the wavelength of the surface acoustic wave; andsaid first space is within the range from about 172λ to about 1.88λ andsaid second space is within the range from about 2.22λ to about 2.33λ.32. A surface acoustic wave filter device according to claim 31, whereinsaid piezoelectric substrate is an approximately 36° to 44°-rotatedY-cut LiTaO₃ substrate made of a LiTaO₃ single crystal with anorientation rotated about the X axis from the Y axis to the Z axiswithin the range from about 36° to about 44°, and at least one electrodefinger is inserted in at least one of the first and second spaces in thefirst and second surface acoustic wave filter elements so that theelectrode covering ratio of the space in which said electrode finger isinserted becomes equal to or greater than about 50%.
 33. A surfaceacoustic wave filter device according to claim 32, wherein saidelectrode covering ratio is equal to or greater than about 63%.
 34. Asurface acoustic wave filter device according to claim 31, wherein firstand second reflectors are disposed on respective opposite sides in thesurface acoustic wave propagation direction of the area in which saidplurality of IDTs of the first surface acoustic wave filter element aredisposed, and third and fourth reflectors disposed on respectiveopposite sides, in the surface acoustic wave propagation direction, ofthe area in which said plurality of IDTs of the second surface acousticwave filter element are disposed, wherein the distance between the firstreflector and the second reflector is substantially equal to thedistance between the third reflector and the fourth reflector.
 35. Asurface acoustic wave filter device according to claim 31, wherein theunbalanced-side terminal of the first surface acoustic wave filterelement and the unbalanced-side terminal of the second surface acousticwave filter element are connected to each other via an electrode patternon the piezoelectric substrate.
 36. A surface acoustic wave filterdevice according to claim 31, further comprising a series resonatorconnected on the unbalanced terminal side.
 37. A surface acoustic wavefilter device according to claim 31, further comprising surface acousticwave resonators connected in series to the respective terminals on thebalanced terminal side.
 38. A surface acoustic wave filter deviceaccording to claim 31, further comprising a ladder-type surface acousticwave filter connected in a cascade fashion on the balanced terminalside.
 39. A surface acoustic wave filter device according to claim 31,further comprising a package which has a cavity for housing a chipincluding the piezoelectric substrate on which the surface acoustic wavefilter elements are disposed, said package having on an inner bottom ofthe cavity electrodes pads to be electrically connected to electrodepatterns on the chip, wherein at least one of the electrode patterndisposed on the piezoelectric substrate, the package, or the electrodepads has a substantially axially symmetric structure.
 40. A surfaceacoustic wave filter device according to claim 39, wherein at least twoof the electrode pattern, the package, and the electrode pads havestructures which are substantially axially symmetric with respect to thesame symmetry axis.
 41. A surface acoustic wave filter device accordingto claim 31, further comprising a package which has a cavity for housinga chip including the piezoelectric substrate on which the surfaceacoustic wave filter elements are disposed, said package having on anouter bottom of the cavity an external input terminal and two externaloutput terminals, such that the two external output terminals aresymmetric with respect to said external input terminal.
 42. A surfaceacoustic wave filter device according to claim 41, further comprising aexternal grounded terminal provided between said two external outputterminals.
 43. A surface acoustic wave filter device according to claim41, further comprising two external grounded terminals provided betweensaid external input terminal and said two external output terminals,respectively.
 44. A duplexer comprising a surface acoustic wave filteraccording to claim
 31. 45. A communication device comprising a duplexeraccording to claim 44.